1. Field of the Invention
The present invention relates to a method for forming a film such as an interlayer insulating film provided between layers of polysilicon or metal having a high melting point, or an interlayer insulating film provided between wiring layers of aluminum.
2. Description of Background Art
Conventionally, for forming an interlayer insulating film on a material to be treated, a CVD method has been used. However, as requirements for reducing the space between wiring layers on a material to be treated are increasing, it becomes difficult to form a sufficient film by using a CVD method. Specifically, since a film formed by a CVD method has a conformal shape, it is not possible to sufficiently fill the small space between wiring layers.
Then, the formation of a SOG coating film on a material to be treated has been examined. However, in a case of forming a SOG coating film using a common open-type cup, a roughness-density difference in film quality occurs between the SOG coating film formed at the bottom of a space portion of a line-and-space pattern portion and the SOG coating film formed in the other portion. Generally speaking, a SOG coating film formed at the bottom of a space portion of a line-and-space pattern portion is rough, while a SOG coating film formed in the other portion is dense.
In this regard, the roughness-density difference refers to a phenomenon that the same SOG coating film has a different density depending upon the portion where the SOG coating film is applied. For example, when the dense portion has a density of 100, the rough portion has a density of 50.
In particular, if the aspect ratio of the space between the wiring layers is high, such a phenomenon of a roughness-density difference gets worse. If the aspect ratio is low, such a phenomenon rarely occurs.
In a case of using a common cup (open-type), it is common that a film is formed by applying SOG only once, even to a high aspect ratio portion. Therefore, it is difficult to solve the problem of a roughness-density difference.
To solve the above problem, in Japanese Unexamined Patent Application Publication No. Hei 7-227568 (corresponding to U.S. Pat. No. 6,277,441 B1), the present inventor has disclosed that a material to be treated is rotated in two stages to apply SOG in which the time interval between the first rotation and the second rotation is made 10 times longer than the time of the first rotation. However, this method cannot overcome the problem of a roughness-density difference. The high aspect ratio portion becomes rough as shown in FIG. 4.
The above-mentioned phenomenon of a roughness-density difference deteriorates reliability of a device due to the rough portion. Specifically, it is fatal to a gate electrode which is the heart of a semiconductor device. In the rough portion, the coating film has fine voids therein. Therefore, when heating (baking) is conducted in a subsequent process after applying SOG, gas in the voids expands, thereby causing cracks to occur in the SOG coating film. In that case, thee SOG coating film cannot function as a SOG insulating film.
Consequently, an object of the present invention is to provide a method for forming an interlayer coating film which is uniform even in a fine space of 0.13 μm or less between wiring layers by reducing an area of a rough portion and a dense portion, and reducing a roughness-density difference.